Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece such as a semiconductor wafer. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity. The ion beam may be distributed over the wafer area in a scan pattern defined by only beam scanning, by only wafer movement, or by a combination of beam scanning and wafer movement.
Introducing the ions at a specified depth and density into the wafers, which may be a uniform depth and density, is important to ensure that the semiconductor device being formed operates within specification. One factor that can affect the uniformity of the dose into the wafer is the ion beam current. However, the ion beam current can have unexpected instantaneous fluctuations both greater than and less than a desired beam current that may adversely affect uniformity requirements. The magnitude and duration of such fluctuations may sometimes be referred to as the “ion beam noise” of the ion beam.
One effective method of addressing ion beam noise is to achieve a target dose by a plurality of incremental applications of smaller dose levels that sum to the target dose to “average out” the ion beam noise for each location on the front surface of the wafer. Therefore, any fluctuations in ion beam current attributable to each of the plurality of incremental applications of smaller dose levels tend to offset each other. For example, the sum of fluctuations in ion beam current greater than the desired beam current tends to offset the total fluctuations in ion beam current less than the desired beam current for a particular location on the front surface of the wafer. Therefore, the sum of the smaller dose levels for each of the plurality of incremental applications can closely approximate the desired target dose. In general, the effectiveness of this method improves as the plurality of incremental applications increases for each location on the front surface of the wafer.
Differing conventional methods exist to increase the plurality of incremental applications or “touches” of the ion beam to the front surface of the wafer. For example, for an ion beam distributed over the front surface of the wafer by a combination of beam scanning and wafer movement, some conventional methods include increasing the number of passes of the wafer by the scanned ion beam, slowing the speed at which the wafer is moved by the scanned ion beam, and increasing the frequency of the scanned ion beam. While all effective, these conventional methods may be optimized for given throughput and other system requirements. Therefore, some recipes may have to place tighter restraints on the level of permissible ion beam noise allowed during set-up of the ion implanter. This could result in longer set-up times and lower throughput.
A conventional scan pattern, for example developed by the relative motion of a scanned ion beam and the speed at which the wafer is moved by the scanned ion beam in one instance, has a constant relative motion direction between the ion beam and the wafer while the ion beam is incident on the wafer. Only when the beam has traveled beyond an edge of the wafer, does the relative motion reverse direction. This process continues until the ion beam is distributed across the desired front surface area of the wafer. Therefore, once other conventional methods to increase the number of incremental applications or “touches” of the ion beam to the front surface of the wafer are optimized, this conventional scan pattern does not provide an additional way to further increase the number of incremental applications to the wafer to lessen the impact of ion beam noise.
Accordingly, there is a need in the art for a new and improved apparatus and method of providing a new scan pattern on the front surface of a workpiece to lessen the impact of ion beam noise in ion implantation.